Radio frequency identification (RFID) systems are used to detect and prevent inventory shrinkage and to perform inventory management functions in a variety of retail establishments, apparel and mass merchandisers, supermarkets, libraries, video stores, and the like. In general, such systems use an intelligent tag which is secured to or associated with an article (or its packaging), typically an article which is readily accessible to potential customers or facility users. The process wherein intelligent tags are secured to or associated with an article (or its packaging) is often referred to as "tagging" the article. In general, such RFID systems are employed for detecting the presence (or the absence) of a unique intelligent tag and, thus, a protected article within a surveilled security area or detection zone, also referred to herein as an "interrogation zone." The detection zone is located at or around an exit or entrance to the facility or a portion of the facility, at the point of sale, or proximate to a hand-held, portable interrogator.
One type of RFID system which has gained widespread popularity uses an intelligent tag which includes a self-contained, passive resonant circuit in the form of a small, generally planar printed circuit which resonates at a predetermined detection frequency within a detection frequency range. A transmitter, which is also tuned to the detection frequency, transmits electromagnetic energy or an interrogation signal into the detection zone. A receiver, tuned to the detection frequency detects amplitude disturbances on the electromagnetic field that are imparted by the intelligent tag. When an article having an attached intelligent tag moves into or passes through the detection zone, the intelligent tag is exposed to the transmitted energy. That is, the intelligent tag is interrogated. The detection of such an output signal by the receiver indicates the presence of an article with an intelligent tag within the detection zone and the receiver activates an alarm to alert appropriate security or other personnel.
One well-known RFID system has a transmitting and detecting frequency in the radio frequency range. The intelligent tags used with such systems are referred to as RF tags or RF intelligent tags. The RF tags associated with each article may be identical so that all articles having an intelligent tag, regardless of article size or value, return an identical signal to the receiver. Alternatively, the RF tags may be passive resonant intelligent tags which return unique identification codes. U.S. Pat. Nos. 5,446,447 (Carney et al.), 5,430,441 (Bickley et al.), and 5,347,263 (Carroll et al.) disclose three examples of such intelligent tags. These intelligent tags typically include an integrated circuit to generate a unique identification code. Such "intelligent" intelligent tags provide additional information about the article detected in the zone of the interrogator. These intelligent tags typically respond to, and transmit signals, in the radio frequency range, and are known in the art as "radio frequency identification (RFID) tags or "intelligent tags." RFID tags are used in RFID systems. intelligent tags may also resonate at non-RF frequency bands, and may be referred generically as "EAS markers."
Existing RFID systems of the type described above and of other types have been shown to be effective in preventing the theft or unauthorized removal of articles.
FIG. 1 shows a conventional transceiver assembly 10 of an RFID system. The assembly includes a pair of spaced pedestal transceiver antennas 12 and 12' which define a detection zone 14 therebetween. In one conventional scheme, transmitter and receiver coils are placed in each of the antennas 12 and 12'. In another conventional scheme, a transmitter coil is placed in the antenna 12 and a receiver coil is placed in the antenna 12'. The maximum size of the detection zone 14 depends largely upon the "read range" of the intelligent tags used in the RFID system. The "read range" is the range in which a passive resonant signal can be accurately detected and discriminated by the signal receiving apparatus.
Passive resonant signals are relatively low power signals and must be discriminated within a relatively noisy environment. There are many sources of potential noise in the detection zone 14, such as signals from other intelligent tags, signals generated by interaction of the transmitter frequencies with metal objects and neighboring electrical equipment. Furthermore, the RFID system itself is a significant source of noise.
In the RFID system, significant amplitude and phase noise are produced from the oscillator used to generate the fundamental field frequency of the transmitter loop antenna. Furthermore, significant amounts of noise travels on the signal path between the transceiver electronics and the circuitry for processing the analog tag signals. This signal path is hardwired in conventional RFID systems. See, for example, U.S. Pat. No. 4,623,877 (Buckens). Ground loops and common mode signals introduce noise along this signal path. Switching noise from digital signal processing (DSP) circuitry for processing the analog tag signals is often found on the signal path. DC power lines which supply power to the transceiver electronics and to the circuitry for processing the analog tag signals also introduce noise on the signal path.
Such noise decreases the signal-to-noise ratio of the tag response signal and thereby limits the read range. The noise problem gets worse as the antennas 12 and 12' are moved further apart, since the tag signal weakens with distance from the receiver antenna, whereas potential environmental noise sources increase with distance from the receiver antenna. Furthermore, governmental authorities, such as the Federal Communications Commission (FCC), regulate radiation emission levels in the frequency range used by RFID systems and do not allow emission levels to exceed maximum predetermined levels. This further limits the strength of the signals allowed in the detection zone, thereby constraining the read range. Many conventional RFID systems currently operate at or near the FCC limits.
Conventional RFID systems experience tag reading problems even when the read range is an acceptable value. For example, the signal from one particular intelligent tag may be missed due to unexpected noise in the detection zone or due to a low signal-to-noise ratio as a result of a weak or significantly attenuated tag response signal. Thus, conventional RFID systems sometimes fail to "hear" small signals, or even normal strength signals when the environment is unusually noisy.
Typically, the spacing between the transceiver antennas 12 and 12' is in the range of from three to six feet depending upon the particular RFID system and the particular application in which the system is employed. However, to avoid inhibiting the entry/exit to a store, it is desirable that the antennas be spaced from each other by at least the width of the entry/exit, which may be six feet or greater in some types of stores (e.g., home centers). It is also sometimes desirable to hide the antenna apparatus. However, it is not feasible to hide the antenna apparatus if the antennas must be a very close to each other to obtain acceptable performance. Thus, antenna placement options are constrained in conventional RFID systems.
One scheme for increasing the read range of an RFID system is described in copending U.S. application Ser. No. 08/783,423, filed Jan. 14, 1998, entitled "Multiple Loop Antenna." However, this scheme addresses the antenna S design, and does not address the problem of noise generated by the RFID system itself, and by other external noise sources.
In sum, there is a significant, long-felt and presently unmet need to improve the detection capabilities and read range of RFID system electronics without violating governmental regulations on field strength.
The present invention fulfills this need by providing methods and systems which significantly reduce background noise produced by external/environmental sources and internal RFID system components, thereby providing an RFID system which has an improved read range, and which can hear weaker signals.